This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Adenosine kinase (AK) is a key enzyme in purine metabolism in parasites, and a potential chemotherapeutic target for the treatment of Toxoplasma gondii infections. The initial structures of unliganded AK and AK in complex with adenosine (or 7-iodotubercidin) and the non-hydrolysable ATP analog AMP-PCP revealed a novel catalytic mechanism. A domain closure triggered by a GG switch upon adenosine binding sequesters the adenosine and [unreadable]-phosphate of ATP from the solvent. The formation of the anion hole induced by the ATP binding completes the structural requirements for catalysis. A later published structure, which was determined to 1.1 [unreadable] resolution, of AK complexed with AMP-PCP provided direct evidence that ATP binding at mM concentrations does not require adenosine binding as a prerequisite. The overall structure of this binary complex is similar to the apoenzyme with an open conformation. AMP-PCP is bound in two relaxed conformations and without anchoring by Arg136. The induced anion hole is the same as that in the ternary complex of AK/adenosine/AMP-PCP.